An ankle prosthesis mainly includes a tibial implant, a talus implant, and a prosthetic skid interposed between the tibial and talus implants. The prosthetic skid is said to be “moving” when it is assembled to the tibial and talus implants so as to be movable relative to both of them, whereas the prosthetic skid is the to be “stationary” when, while still being movable relative to the talus implant, it is secured in stationary manner to the tibial implant.
During a surgical intervention by an anterior approach path, putting a moving-plate or a stationary-plate prosthesis into place in the ankle of a patient requires the bottom end of the tibia and the top end of the talus of the patient to be prepared, in particular by resections, so as to make it possible to secure the tibial and talus implants permanently thereto. In practice, once these bone preparation operations have been performed, the surgeon frequently makes use of test components or “phantoms” generally corresponding to at least some aspects of the corresponding prosthetic implants, thus making it possible to verify that the preparation operations are appropriate and that there is no need for additional bone removal or additional resurfacing.
Thus, the surgeon puts into place, in the ankle of the patient: a phantom corresponding to the talus implant, which phantom is held in place on the resected end of the talus; a phantom corresponding to the tibial implant, which phantom is held in place by being pressed against the resected end of the tibia; and a phantom skid corresponding to the prosthetic skid, which phantom skid is interposed between the talus and tibial implant phantoms and movable relative to each of the implant phantoms.
The surgeon typically has a plurality of phantom skids available of different respective thicknesses, with each of the phantom skids corresponding to a prosthetic skid suitable for being implanted subsequently. The surgeon selects the phantom skid having the thickness that is the most appropriate. Depending on the morphology of the patient, on the pathology that requires a prosthesis to be put into place, and/or on the relative positioning of the resections performed, the thickness of the skid that will actually be implanted can vary (i.e. its dimension in a generally vertical direction can vary), so that in practice the surgeon has available a series of several prosthetic skids each presenting substantially the same moving or stationary connection surfaces relative to the talus and tibial implants, but with respective thicknesses equal to about 4 millimeters (mm), about 5 mm, about 6 mm, and about 7 mm, for example.
After the above-mentioned phantom components have been put into place, the surgeon moves the ankle joint of the patient, in particular with flexion-extension movements. The surgeon can then act, in preoperative mode, to verify the quality with which the bones have been prepared, and also the dynamic behavior of the ankle provided with the phantom components, which is representative of the dynamic behavior the ankle will subsequently have, once fitted with the prosthetic components that are to be implanted.
In spite of the structural qualities of ankle prostheses, clinical hindsight shows that such ankle prostheses are implanted quite frequently in unsatisfactory manner. For example, for a stationary skid prosthesis, significant excess stress is to be observed in service between the skid and the tibial implant, which leads in the fairly short term to the stationary connection zone between the implant and the skid breaking, or to damage to the end of the tibial bone. With regard to moving skid prostheses, which specifically tend to be used to overcome the above-mentioned drawback of stationary skid prostheses, it is found in service that the top surface of the prosthetic skid extends to a greater or lesser extent outside the peripheral outline of the bottom surface of the tibial implant, such that the portions of these surfaces that are in moving contact are smaller than intended, leading to premature wear of the skid and/or of the implant. In other words, the insertion methods currently in use do not guarantee effective relative positioning between the tibial implant and the prosthetic skid, regardless of whether the skid is stationary or moving, since no reliable correction option is made available to the surgeon.